An olefin low molecular weight polymer such as a polyethylene wax is used in applications such as a pigment dispersant, a resin processing aid, a printing ink additive, a paint additive, a rubber processing aid and a fiber treating agent. In addition, an olefin low molecular weight polymer is also used in a releasing agent for toner. Recently, from a viewpoint of energy saving, a low temperature fixing toner is sought, and an appearance of a wax having better release characteristics at a low temperature, that is, a wax having a lower melting point with the same composition and the same molecular weight is desired.
As a process for preparing such an olefin low molecular weight polymer, conventionally, a titanium catalyst has been usually used industrially. However, although the catalyst has an advantage that, in case of using the catalyst system, the yield of the low molecular weight polymer per unit amount of the catalyst is high, and the productivity is high, it also has a technical challenge that it is necessary to maintain a high hydrogen partial pressure in the gaseous phase in the polymerization system and, as a result, a large amount of alkanes as side products are produced.
Furthermore, the molecular weight distribution of the resulting low molecular weight polymer is wide and, in particular, since the low molecular weight polymer having a molecular weight of 1000 or smaller is greatly sticky, it is difficult to use it in the aforementioned applications without removing the low-molecular weight fraction.
As a method for solving these challenges, Japanese Patent Laid-Open Publication No. 210905/1984 proposes a process for preparing a low molecular weight polymer using a vanadium catalyst. This publication describes that a low molecular weight polymer having a narrow molecular weight distribution can be prepared under a low hydrogen partial pressure, as compared with that prepared using a titanium catalyst. However, the molecular weight distribution is not necessarily sufficient. In addition, Japanese Patent Laid-Open Publication No. JP-A No. 78462/1985 proposes a process for preparing an ethylene wax, comprising the step of polymerizing ethylene or copolymerizing ethylene with an α-olefin, in the presence of a metallocene catalyst comprising (A) a compound of a transition metal selected from the group consisting of Group 4 elements, Group 5 elements and Group 6 elements of the periodic table and (B) an aluminoxane. According to this process, an ethylene wax having a narrow molecular weight distribution can be prepared, but a process for preparing an ethylene wax having further excellent productivity is desired.
Furthermore, Japanese Patent Laid-Open Publication No. 203410/1989 and Japanese Patent Laid-Open Publication No. 49129/1994 describe preparation of an ethylene wax using a metallocene catalyst comprising a metallocene and an aluminoxane. However, in these methods, the productivity is not necessarily sufficient. When a polymerizing temperature is raised, it becomes easy to remove the polymerization heat, and the productivity can be improved, but there is a challenge that a yield of a low molecular weight polymer per unit weight of a catalyst is lowered.
Japanese Patent Laid-Open Publication No. 239414/1996 has already proposed a process for preparing an ethylene wax, comprising the step of (co)polymerizing ethylene in the presence of an olefin polymerization catalyst comprising (A) a Group 4B transition metal compound containing a ligand having a cyclopentadienyl skeleton, (B) a compound which reacts with the (A) to form an ion pair, and (C) an organoaluminum compound. According to this process, an ethylene wax having a narrow molecular weight distribution can be prepared at high production efficiency, but a process for preparing an ethylene wax further excellent in the productivity and the quality is desired.